1. Field
Embodiments generally relate to geographical terrain modeling.
2. Background Discussion
Digital Terrain Models (DTMs) are used for many applications in Geographical Information Systems (GIS) ranging from three dimensional (3D) terrain viewing applications to orthorectification (the geometric scaling of aerial photographs so they can be used as maps) and flood plain analysis. A DTM is a data file containing the elevation of the bare Earth (the terrain) at various points on the Earth's surface. As an example, elevations in a DTM can be specified in a square grid of cells with the grid represented in a coordinate system such as latitude and longitude. The elevation of terrain in a cell can be represented by an elevation value associated with the cell.
DTMs are commonly built with data gathered by remote sensing techniques such as photogrammetry or LiDAR (Light Detection And Ranging). In addition to terrain elevations, these techniques also produce the elevations of features such as vegetation, buildings, bridges and road overpasses that are on or above the terrain. Since terrain elevations in DTM cells under a feature are occluded by the feature, various processing steps are used to locate the features and estimate the terrain elevations in the occluded cells. The estimation techniques might use valid terrain elevations in the neighborhood of the feature together with surface fitting (e.g., a Laplacian surface) to estimate the occluded terrain elevation values. In some cases, which could occur, for example, when the occluded terrain has sharp features and elevation changes rapidly, these estimates are not accurate enough and manual editing of estimated terrain elevation values is needed to achieve desired accuracy. This manual editing increases the cost of producing DTMs. Furthermore, because the estimates are not sufficiently accurate, features rendered using these estimates appear to merge with surrounding terrain gradations. This degrades user viewing experience. In other applications (e.g., erosion modeling, flood plain analysis) inaccuracies in estimated elevations can cause erroneous results.